This invention relates to technology of interconnecting a plurality of LAN's (Local Area Networks) and more particularly to a technique for transmission of a communication frame in a network including a plurality of LAN's having different types of protocol.
Conventionally, in connection with a network system capable of performing communications between a plurality of terminals having different types of protocol, a communication method has been known and disclosed in, for example, JP-A-62-49748, according to which a protocol converter is provided between networks, and a communication frame (or a message) from one network accommodating a plurality of terminals having different types of protocol is converted by the protocol converter into a frame of common protocol which in turn is sent to the other network.
Also, JP-A-60-74842 discloses a communication method wherein in a network system comprising a network accommodating terminals applied with upper level protocol and a second network accommodating terminals applied with lower level protocol and connected to the former network through gateways, each gateway is provided with a table memory which stores the types of protocol to be applied in correspondence to addresses of all destination terminals, and each gateway determines applicable protocol by looking up the table on the basis of a destination address contained in a received frame so as to convert the protocol of the received frame.
A connection configuration of a network having a plurality of LAN's has also been known wherein a plurality of LAN's each accommodating terminals and operable at a relatively low speed (hereinafter referred to as secondary LAN's) are connected to a LAN of large capacity operable at a relatively high speed (hereinafter referred to as primary LAN) through coupling units (hereinafter referred to as bridges), and communication between a terminal belonging to a secondary LAN and another terminal belonging to a different secondary LAN is carried out through the medium of the primary LAN. When a system incorporating the technique disclosed in the aforementioned JP-A-62-49748 is applied to the above network system using the primary LAN in making an attempt to connect a plurality of secondary LAN's applied with different types of protocol to the primary LAN so as to perform communications regardless of the difference in the type of protocol, a bridge receiving a frame from an associated secondary LAN is required to convert the received frame into a frame of common protocol and transmit the converted frame to the primary LAN and conversely, when receiving a frame from the primary LAN, the bridge is required to convert the received frame into a frame of protocol of the associated secondary LAN and transmit the converted frame thereto. In this combination system, however, two types of protocol conversion i.e., conversion to the common protocol and conversion to the secondary LAN protocol are needed even in communications which are carried out between terminals of the same protocol and which does not require a protocol conversion, thereby raising a problem that the overhead is increased.
In another combination system in which the technique disclosed in the aforementioned JP-A-60-74842 is alternatively applied, a bridge receiving a frame from an associated secondary LAN decides the type of protocol used by a destination terminal by looking up a destination address contained in the received frame. To this end, each bridge is required to hold a table showing the relation between terminals and a protocol applied thereto. After recognizing the type of protocol, the bridge converts the received frame into a frame of protocol used by the destination terminal and transmits the converted frame to the primary LAN. A bridge receiving the frame subject to the protocol conversion from the primary LAN can transmit the received frame directly to an associated secondary LAN because the received frame matches the protocol of the associated secondary LAN. This combination system however presupposes that each bridge can perform protocol conversion for all of the destination terminals, and each bridge is required to hold information about all terminals in the LAN interconnecting network system, thus raising problems that the memory overhead is increased and the table management is complicated when changing terminal addresses.
Another example of a communication method in the aforementioned network using a plurality of secondary LAN's interconnected through a single primary LAN has been known as disclosed in, for example, JP-A-60-10839, according to which respective gateways (or bridges) connecting respective secondary LAN's to the primary LAN are provided with an address conversion table, and each gateway looks up the table on the basis of a destination address contained in a frame received from an associated secondary LAN to extract from the table an address of a destination bridge connecting a secondary LAN, to which a destination terminal belongs, to the primary LAN and then transmits the received frame, now including the destination bridge address, to the primary LAN. The destination bridge receives the frame from the primary LAN and removes the bridge address from that frame to produce the rest which in turn is sent to the associated secondary LAN. This communication method of JP-A-60-10839 presupposes that all of the secondary LAN's connected to the primary LAN have the same protocol, in other words, it does not consider a network in which secondary LAN's having different types of protocol coexist. Thus, in this prior art arrangement, each bridge simply adds the destination bridge address to the received frame from the associated secondary LAN or removes the destination bridge address from the frame received from the primary LAN and therefore when different types of protocol are applied to the source secondary LAN which originates the frame and the destination secondary LAN, there still remains a problem that a frame of different type of protocol interferes with a destination secondary LAN to cause an erroneous operation.